Failure Analysis: Motor-Pump Unit Shaft

Failure Analysis: Motor-Pump Unit Shaft

Project Description

A sanitary plant customer brought this motor-pump unit in for repair in 2014. It had been used to move liquid before it failed.

The shaft, which was shared between the motor and pump, had failed near the seal. Our observations included:

  • A fair amount of corrosion on the surfaces of the flange mount, pump impeller and fractured surfaces of the shaft — but not on surfaces beyond the seal

  • Ratchet markings around the peripheral of the fractured surfaces

  • An oval-shaped instantaneous failure zone

The markings observed on the fractured faces indicated that the shaft had been under a combined load condition of rotational bending and tension. The shaft failed as result of fatigue, initiated by corrosion of the shaft.

Upon further investigation, it was discovered that the shaft had previously been rebuilt elsewhere with the wrong material, which allowed for the corrosion to occur.